By looking at penal law, we see, that simple nudity is not a criminal offence: In penal law, all offences, that may be related to nudity, concern sexual deeds in public, only. Conclusion: In Germany, penal law is not applicable to mere nudity (as in nude sports, nude hikes etc.).
Terms such as "public disturbance" or "exhibitionism", which uninformed fellow citizens willingly cite, solely concern sexual deeds and have absolutely nothing to do with nudity in terms of naturist activities. Unfortunately, these uninformed citizens often cause with their unjustified calls, that police and regulatory authorities are distracted from their real duties.
The basis for the penal law in Germany is the German Strafgesetzbuch (StGB) – in English: Penal Code. The Penal Code contains two sections, which address sexual delicts.
We have discovered a judgment, cited in de.wikipedia.org, with a definition, which aspects need to be fulfilled in order to assess a sexual deed as exhibitionism:
Judgment as to section 183 "Exhibitionism", Bavaria [16 June 1998]
(Translated cit. of a statement in an article "Exhibitionismus" in de.wikipedia.org)
»According to a judgment of the Bayerisches Oberlandesgericht [en: Bavarian Higher Regional Court] in 1998 (BayObLG (2. Strafsenat), Urteil vom 16. Juni 1998 – 2 St RR 86/98, NJW 1999, 72), the element of an offence of exhibitionism is only fulfilled, if the disrobement serves for sexual satisfaction. Nudity and therefore nude sport in the public alone is therefore not punishable in Germany.«
Source (in German): Exhibitionismus (de.wikipedia.org)
Also, there is an article "Exhibitionism" written in English in en.wikipedia.org, but it is not a translation of the German one, which addresses especially the situation in Germany!
– This judgment is cited in D · Penal law.